1. Field of the Invention
The invention relates to a device for measuring a fill level of a liquid in a container, more specifically a fuel tank of a motor vehicle, the device having an ultrasonic sensor for generating ultrasonic waves in the liquid and for receiving the ultrasonic waves reflected in the region of the liquid level in the container and an evaluation unit for calculating the fill level of the liquid from the echo time of the ultrasonic waves received by the ultrasonic sensor.
2. Description of the Related Art
A device for measuring the fill level of fuel in a fuel tank using an ultrasonic sensor is known in which the ultrasonic sensor has an outside diameter corresponding to the inside diameter of a measuring tube arranged in the fuel tank. The ultrasonic sensor is plugged into an end of the measuring tube. The measuring tube penetrates the bottom of the container and is filled with the liquid present in the fuel tank via compensating bores. To amplify the ultrasonic waves reflected by the liquid level, a float for reflecting the ultrasonic waves is arranged in the measuring tube.
A disadvantage of the known device is that the echo time of the reflected ultrasonic waves depends very strongly on the composition of the liquid and on its temperature. The device provided for use in the fuel tank of a motor vehicle, in particular, is very inaccurate because the temperature of the fuel varies over a large range such as, for example, upon refilling or upon parking in the sun.
One solution is to measure the temperature in the tank and to correct the echo time of the ultrasonic waves in accordance with the temperature and material characteristics of the liquid. However, this requires a complex calculation of the fill level. Moreover, in the case, for example, of diesel fuel it is not possible to correct a change in the composition of diesel fuel which occurs as better fuels are developed or a change in composition associated with water absorption.
The object according to the present invention is to provide a device for detecting the fill level of liquid in a container via reflections of an ultrasonic wave detected by a sensor, wherein the sensor provides a reliable detection for liquids of varying temperature and composition.
The object according to the present invention is met by a measuring reflector arranged in a bottom region of the container and at a designated spacing upstream of the ultrasonic sensor and an evaluation unit operating arranged for comparing the echo times of the ultrasonic waves reflected in the region of the liquid level and of the ultrasonic waves reflected by the measuring reflector.
This configuration allows a change in the echo time of the ultrasonic waves reflected in the region of the liquid level through temperature variations or variations in the composition of the liquid to be easily accounted for by comparison with the echo time of the ultrasonic waves reflected by the measuring reflector. The fill level of the liquid in the container may be specified., for example, as a function to a ratio of the echo times and the spacing of the ultrasonic sensor from the measuring reflector. Since the ratio of echo times is independent of the composition of the liquid and its temperature in the container, the fill level is determined particularly accurately. A further advantage of this configuration is that a single device may be produced and mounted without calibration for any desired liquid. This reduces production costs of the device, particularly when the device comprises fuel tanks of motor vehicles and large numbers of the devices are mass produced.
A particularly accurate fill level may be determined using the device according to the invention when the measuring reflector is moveable from a lateral position at the designated spacing upstream of the ultrasonic sensor. The determination of the echo time of the reflection from the measuring reflector and of the echo time of the reflections from the liquid level is thereby performed sequentially. Therefore, the reflections do not mutually influence or interfere with one another.
The device having a movable reflector may comprise a simple design such as, for example, a slide which can be moved into the designated position or a flap. The movement of the slide or the flap may be effected mechanically, electrically, hydraulically, pneumatically, or magnetically.
The device according to another embodiment of the present invention requires no moving parts when the ultrasonic sensor comprises two sensor elements in which one of the sensor elements is arranged for determining the echo time of the reflections of the measuring reflector and the other of the two sensor elements is arranged for determining the echo time of the reflections from the liquid level. This arrangement of the two sensor elements also prevents mutual influencing of the reflections.
The device according to the present invention may be cost-effectively produced when the measuring reflector is arranged on a splash pot bearing against the bottom of the container. Since the splash pot is located at the deepest point of the container, it is easy to ensure that a sufficient amount of liquid is always present on the section between the ultrasonic sensor and measuring reflector.
In accordance with another advantageous embodiment of the present invention, disturbing reflections may easily be avoided when the ultrasonic sensor is fastened on the outside of the container and is arranged opposite a measuring tube arranged in the container. The measuring tube can thereby be guided from the lower up to the upper wall of the container. The measuring tube may also bear the measuring reflector.
The device according to the present invention requires minimal components for mounting when the measuring tube comprises a stepped design with the measuring reflector arranged on the lowermost step. In this embodiment, a single ultrasonic sensor receives the two reflections of the measuring reflector and the liquid level. Separation of the signals is then performed in the evaluation unit.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.